1. Field of the Invention
The present invention relates to a phase compensator for compensating for a phase of light having a particular wavelength among a plurality of wavelengths of light and a compatible optical pickup using the phase compensator.
2. Description of the Related Art
In an optical recording and/or reproduction system for recording information on and/or reproducing recorded information from an optical disc using a light spot focused by an objective lens, the recording density is determined by a size of the light spot. The size of the light spot (S) is proportional to the wavelength of light (λ) and is inversely proportional to a numerical aperture (NA) of the objective lens, as shown by expression (1) below:S∝λ/NA  (1)
Accordingly, to reduce the size of the light spot being focused on the optical disc for a higher recording density, a short-wavelength light source, such as a blue-violet laser, is adopted along with an objective lens having an NA of 0.6 or greater. Denoting a tilt angle of the optical disc as θ, a refractive index of the optical disc as n, a thickness of the optical disc as d, and a numerical aperture of the objective lens as NA, coma aberration W31 occurring due to tilting of the optical disc is expressed by expression (2) below:
                              W          31                =                                            -                              d                2                                      ·                                                                                n                    2                                    ⁡                                      (                                                                  n                        2                                            -                      1                                        )                                                  ⁢                sin                ⁢                                                                  ⁢                θ                ⁢                                                                  ⁢                cos                ⁢                                                                  ⁢                θ                                                              (                                                            n                      2                                        -                                                                  sin                        2                                            ⁢                      θ                                                        )                                                  5                  2                                                              ⁢                      NA            3                                              (        2        )            
In expression (2), the refractive index n and the thickness d of the optical disc refer to those of an optical medium, i.e., an optical disc substrate, positioned between a light incident surface and an information recording surface of the optical recording medium.
In consideration of expression (2) above, to ensure tolerance with respect to the tilt of an optical disc, there is a need to reduce the thickness of the optical disc as the NA of the objective lens is increased for a higher density. The thickness d of the optical disc is reduced from 1.2 mm for CDs to 0.6 mm for DVDs. Also, there is a high possibility that the thickness d of next generation DVDs (so-called high-definition (HD)-DVDs) will be determined to be 0.1 mm. The NA of the objective lens was increased from 0.45 for CDs to 0.6 for DVDs and an NA of about 0.85 for HD-DVDs is expected. HD-DVDs are highly likely to adopt a blue-violet light source to accomplish the desired recording density. In developing a new standard for optical discs, compatibility with existing optical discs is an important consideration.
For example, in conventional once-recordable optical discs such as a DVD-R and CD-R, reflectivity greatly decreases depending on the wavelength of light. Accordingly, a light source having a wavelength of 650 nm is used for the DVD-R and a light source having a wavelength of 780 nm is used for the CD-R. Therefore, in view of compatibility with the DVD-R and CD-R, three light sources having different wavelengths are required for an optical pickup for HD-DVDs.
A compatible optical pickup using a plurality of light sources of different wavelengths may employ a single objective lens, in view of advantages in size, assembly, and cost. While recording and/or reproducing information using a single objective lens compatible for optical discs of different thicknesses, spherical aberration occurs due to the difference in thickness of the optical discs. Therefore, an aberration compensator is necessary for the compatible optical pickup using the single objective lens to allow satisfactory information recording and/or reproduction with respect to the three different optical discs.